Electron transport through photosystem II, measured as oxygen evolution (OE), was investigated in isolated thylakoid membranes treated with beta-cyclodextrin (beta-CD, a cyclic oligosaccharide constituted of seven alpha-D-glucose residues linked by alpha-1,4 glycosidic bonds) and irradiated with white light of variable intensity. First, we found that the light-response curves of oxygen evolution are well fitted with a hyperbolic function, the shape of which is not affected by the beta-CD concentration. Second, we showed that under conditions of irradiation with white light of saturating intensity (similar to 5000 mu mol of photons/m(2.)s) beta-CD enhances the oxygen evolution in the thylakoid membranes according to a sigmoid function displaying a sharp inflection point, or transition. Unexpectedely, this beta-CD effect is not observed at irradiances of less than similar to 300 mu mol of photons/m(2.)s. We attempted a theoretical analysis of the combined effect of irradiance and beta-CD concentration on oxygen evolution (OEth). For this purpose, the effect of irradiance (I) was modeled with a hyperbola (i) and the beta-CD concentration (C) contribution with a Hill equation, that is, a sigmoid function (ii). The mathematical simulations generated the following general expressions: (i) OEth = [OEmax(0) G(1)(C)]I/[L-1/2(0) G(2)(C) + I] and (ii) G(i)(C) = 1 + p[C-n/(K-1/2(n) + C-n)], where OEmax(0) is the OE maximum (OEmax) in the absence of beta-CD, L-1/2(0) is the photon flux density giving OEmax/2 in the absence of beta-CD, G(1)(C) or G(2)(C) is obtained from G(i)(C) where i is 1 or 2, n is the Hill coefficient, p is a parameter to account for the beta-CD-mediated maximum OE increase, and K-1/2 is the beta-CD concentration giving half-maximal OE activity. The results of the calculations yielded the expression (iii) OEth = 151[1 + 3.3C(4.8)/(13.1(4.8) + C-4.8)]I/{97.5[1 + 5.2C(7.8)/ (14.8(7.8) + C-7.8)] + I} which agrees well with the experimental data for a broad range of I and C. Note that, for C = 0, eq iii reverts to the light-response curve of oxygen evolution in the absence of beta-CD. We conclude that eq iii is a good approximation of the combined effect of irradiance and beta-CD concentration, meaning that the model has a significant value for predicting the outcome of associated photochemical and biochemical reactions.